Electrostatic photography is characterized by simplicity, rapidity, and a moderate price, and a liquid developer is advantageous in that it provides a high image quality. In order to obtain the advantages of both, the liquid developer should be such that it keeps the image quality and sensitivity with a minimal change even after repeated use. Usually, when a liquid developer is used repeatedly, the concentration of toner particles decreases and the developer changes in composition. This causes changes in image quality such as a decrease of image density, and change of gradation, and an increase of sensitivity of the developer. The change of sensitivity makes it necessary to adjust exposure. This detracts from the merits of electrophotography such as simplicity and rapidity. In addition, it becomes necessary to replace the developer frequently if the developer causes changes in the image quality or changes sensitivity to a large extent shortly after repeated use.
Since the currently available liquid developer greatly changes the image quality after repeated use, the developing unit is provided with an automatic control apparatus that maintains the same concentration of toner particles in the developing tank. However, the use of such an apparatus increases equipment cost, makes the equipment complex and vulnerable to troubles, and causes the equipment to need more maintenance. Thus, there has been a demand for a liquid developer that can be used repeatedly without cansing above-described problems.